[ty] Make auto-import ignore symbols in modules starting with a _

This applies recursively. So if *any* component of a module name starts
with a `_`, then symbols from that module are excluded from auto-import.

The exception is when it's a module within first party code. Then we
want to include it in auto-import.

crates/ty_python_semantic/src/module_resolver/path.rs

@@ -594,7 +594,7 @@ impl SearchPath {
         )
     }
 
-    pub(crate) fn is_first_party(&self) -> bool {
+    pub fn is_first_party(&self) -> bool {
         matches!(&*self.0, SearchPathInner::FirstParty(_))
     }